The network stack is designed such that the absence of a function pointer
for promisc or multicast means "unsupported." We should do that instead of
adding functions that just throw errors.

This is necessary for snoopy and promisc mode. Promisc for devether is
both "give me all packets" (aka, actually promiscuous) and "give me packets
I sent." snoopy requires the latter, even if the NIC can't support the
former.